In response to certain wavelengths of electromagnetic radiation (or “actinic radiation”), photochromic compounds, such as indeno-fused naphthopyrans, typically undergo a transformation from one form or state to another form, with each form having a characteristic or distinguishable absorption spectrum associated therewith. Typically, upon exposure to actinic radiation, many photochromic compounds are transformed from a closed-form, which corresponds to an unactivated (or bleached, e.g., substantially colorless) state of the photochromic compound, to an open-form, which corresponds to an activated (or colored) state of the photochromic compound. In the absence of exposure to actinic radiation, such photochromic compounds are reversibly transformed from the activated (or colored) state, back to the unactivated (or bleached) state. Compositions and articles, such as eyewear lenses, that contain photochromic compounds or have photochromic compounds applied thereto (e.g., in form of a photochromic coating composition) typically display colorless (e.g., clear) and colored states that correspond to the colorless and colored states of the photochromic compounds contained therein or applied thereto.
After exposure to actinic radiation for an extended period of time, photochromic compounds can be prone to fatigue, which typically results in a decrease in colorability or optical density and a corresponding increase in yellowness of the photochromic compound when in the activated (or colored) state. The fatigue of a photochromic compound can be characterized as a function of time, which can be quantified as the fatigue rate for a particular photochromic compound. Typically, a lower fatigue rate is associated with an increased usable lifetime for a photochromic compound. Photochromic compounds can be expensive, and articles, such as ophthalmic lenses into which photochromic compounds are incorporated can correspondingly be more expensive than comparative articles that are free of photochromic compounds. As such, it is generally desirable to use photochromic compounds having low fatigue rates, so that the usable lifetime of an article, into which the photochromic compound is incorporated, can be increased or maximized.
It would be desirable to develop new photochromic materials that have desirably reduced fatigue rates. In addition, it would be desirable that such newly developed photochromic materials provide a desirable level of coloration or optical density when in an activated (or colored) state.